Primal meat cuts are generally smaller than a side of beef, but larger than the ultimate cut sold to the retail consumer. Primal cuts are prepared at the slaughter house and shipped to retail meat stores or restaurants where they are butchered into smaller cuts of meat commonly referred to as sub-primal meat cuts. Sub-primal cuts, however, may also be prepared at the slaughterhouse.
Primal and sub-primal cuts are packaged to minimize air (i.e. oxygen) exposure and prevent meat spoilage and discoloration during shipping and handling. One common way to package primal and sub-primal cuts and protect them from the adverse effects of air is to shrink package the cuts in a film having good oxygen barrier properties. For example, a multilayer film having a barrier layer containing polyvinylidene chloride (PVDC) is a well-known packaging material. PVDC is known to have excellent oxygen barrier properties. The other layers of the multilayer film function to protect the PVDC layer and provide the requisite low temperature and abrasion resistance properties, which the PVDC layer lacks. There are other types of biaxially oriented heat shrinkable multilayer films, which do not require oxygen barrier properties, for example films to be used for shrink wrapping poultry.
Shrinkage properties may be produced in a film by biaxially stretching the film in the machine and transverse direction. The resulting film will heat shrink within a specified range of percentages such as from about 20 to about 50 percent at about 90° C. Preferably the shrinkage should be more than 30 percent in at least one of machine and transverse directions at 90° C.
A multilayer heat shrinkable film may also be formed into packaging bags. Bags are generally made by transversely sealing a tubular stock of multilayer film and cutting or splitting the tube, or by superimposing flat sheets of multilayer film and sealing on three sides, or by end folding flat sheets and sealing the two sides. A common type of seal used in manufacturing bags is a hot bar seal. The adjacent thermoplastic layers, referred to as the inner layers, are sealed together by application of heat and pressure across the area to be sealed, using contact with opposing bars of which at least one is heated to cause the adjacent layers to fusion bond. Bags manufactured from a tube stock generally require one hot bar seal transverse to the tube. This seal is also referred to as a bottom seal. Once the bottom seal is applied, the tube stock is transversely cut to form the mouth of the bag. The strength of seals may be measured by determining the time for a seal to fail when under certain conditions. For example, the seal is immersed in hot water at 95° C. and the hot water seal strength (“HWSS”) may be measured by a test such as that described as the “restrained shrinkage-seal strength test” in Funderburk et al U.S. Pat. No. 3,900,635.
Once meat or poultry is inserted into the bag, the package is evacuated and the bag mouth must be sealed. At one time, the standard method for sealing a bag was to fasten a clip around the mouth of the bag. Today, impulse heat sealing techniques are employed to seal the bag mouth. I n general, a bag mouth is impulse sealed by application of heat and pressure using opposing bars. At least one of the bars has a covered wire or ribbon through which electric current is passed for a very brief time period (hence the name “impulse”) to cause the adjacent film layers to fusion bond. Following the impulse of heat the bars are cooled while continuing to hold the bag inner surfaces together to achieve adequate seal strength.
Irradiation of a multilayer film causes the various irradiated layers in the film to cross-link. Under controlled conditions, cross-linking by irradiation not only provides a higher temperature impulse heat sealing range, but also enhances the puncture resistance of the film. Under controlled conditions, polymeric multilayer films can be irradiated to produce a cross-linked product having greater puncture resistance characteristics and other enhancements.
Three basic requirements of a commercial shrinkable multilayer film are as follows                1. Excellent optical properties, such as low haze and high gloss, both before and after shrinkage. This is important for the better aesthetic presentation of the product.        2. Shrinkage at least as high as 30 percent in at least one direction at temperatures about 90° C. This also contributes to better presentation of the product to the consumer (in super markets).        3. Easy bag making of the product (reduced scrap during bag making). This is more easily achievable when the film is comparatively stiff.        
Prior art films typically used may have low haze and high gloss in unshrunk configuration, but much worse optical properties after shrinking. Alternatively, they may keep their optical properties after shrinkage, but do not shrink enough for an acceptable product presentation.
Therefore, it is an object of the present invention to provide a multilayer heat shrinkable film, which optimizes the clarity-shrinkage behaviour. It is a further object of the present invention to render the bag making process more efficient than with existing films.
These objects are solved by the subject-matter of the independent claims. Preferred embodiments are set forth in the dependent claims.